14 SMITHSOXIAX MISCELLANEOUS COLLECTIONS VOL. 92 



trical discharge of cell colloids, and the permeability of the plasma 

 membrane and other physiological processes are at work in a culture. 

 There is a certain range of pH in which each plant can exist. In 

 the fourth and fifth experiments the pH of the cultures was measured 

 at the beginning and at the end of the experiment with the Hellige 

 comparator, which uses colored glass disks in place of the standard 



Table 2. — />// Determinations at End of Experiments 



solutions with which the sample of culture solution plus the indicator 

 is compared. Inadvertently, the cultures grown on the south side of 

 the water baths for the fourth experiment were discarded before the 

 pH could be determined. In these experiments, as shown by table 2, 

 the change in acidity of the cultures at the end of the experimental 

 period as compared with the original acidity is negligible, the maxima 

 being 0.6 pH in the fourth experiment and 0.4 pH in the fifth experi- 

 ment. 



THE NEPHELOMETER 



A special type of nephelometer was constructed to compare the 

 relative concentrations of the solutions. As shown in plate i, figure 2, 

 this piece of apparatus consists of two stationary glass cells in each 

 of which is inserted a similar movable cell filled with distilled water 

 enclosing a stationary glass plunger lined with black paper to prevent 

 reflections. A beam of light is thrown on the glass cells from a con- 

 densing lens, and the cells are adjusted so that the light beam always 

 passes through the same depth of liquid. Each movable glass cell is 

 attached to a metric scale that gives the depth of the solution and is 

 adjustable so that the depth of the unknown solution placed in the 

 bottom stationary cell may change from zero to the length of the 



